Metallic nanoparticles have generated significant scientific and technological interest because of their unusual optical properties, as well as their novel chemical and catalytic properties. Nonspherical nanoparticles, and in particular anisotropic particles, are of major interest because they allow investigation of how shape affects the physical and chemical properties of such structures. Accordingly, a variety of nanoparticle shapes, including stars, cubes, rods, discs, and prisms, have been prepared, and their properties have been preliminarily characterized.
In addition to nanoparticle shape, size is an important nanoparticle parameter because size allows control over several nanoparticle physical and chemical properties, including luminescence, conductivity, and catalytic activity. Over the past century, colloid chemists have gained excellent control over particle size for several spherical metal and semiconductor compositions. This chemical control over particle size led to the discovery of quantum confinement in colloidal nanocrystals and their exploitation as probes in biological diagnostic applications, LED materials, lasers, and Raman spectroscopy enhancing materials.
In contrast, the challenge of synthetically controlling particle shape has been met with limited success. Nevertheless, some physical and solid-state chemical deposition methods for making semiconductor and metal nanowires, nanobelts, and dots have been developed, and a variety of methods for preparing rods using electrochemical and membrane-templated syntheses with a moderate control over aspect ratios now exist.
Several methods of synthesizing silver and gold nanoparticles in a variety of shapes, including disks (Refs. 1-4), rods (Refs. 5-9), prisms (Refs. 10-14), wires (Refs. 15-18), hollow structures (Refs. 19-22), and branched particles (Ref. 23) have been disclosed. Recently, an intense effort has been directed to the synthesis of triangular silver nanoprisms (Refs. 13-17), in part, because of their unusual optical properties, but also because high yield photochemical methods have been developed for preparing relatively monodisperse nanoprisms with significant control over edge length (Refs. 10 and 25). These capabilities allow investigators to make important structure versus property correlations for such nanoparticles.
In general, two approaches are available for synthesizing silver nanoprisms, i.e., a thermal approach and a photochemical approach. Photochemical routes provide more monodisperse nanoprisms and a greater control over structural parameters through selective plasmon excitation of prism precursors and the resulting prisms (Refs. 10-12). Through a judicious selection of irradiation wavelengths, investigators can control the size, shape, and size distribution (i.e., unimodal or bimodal size distributions) of silver nanoprisms. Thermal routes to silver nanoprisms typically involve the gradual conversion of colloidal silver nanoparticles to silver nanoprisms (Refs. 12, 13). No current thermal method provides a rational control over silver nanoprism thickness or size distribution.
For example, methods of manufacturing nonspherical particles, such as triangles and cubes, exist, e.g., platinum cubes and pyramids (Ref. 26), and PbSe, CdS, and Ni triangles (Refs. 27-29). Additional research resulted in methods of synthesizing BaCrO4, CdSe, and Co nanorods and distributions of arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals (Refs. 30-33). Each of these solution methods are based on thermal processes, and in most cases, with the exception of rods, yield relatively small quantities of the desired particle shape.
Synthetic methods that allow control over particle shape are expected to lead to important fundamental and technological advances in the art. This is analogous to particle size control in nanoscale materials which led to the discovery of new and important fundamental science and technological applications in diagnostics, optics, catalysis, and electronics. Therefore, the development of bulk solution synthetic methods that offer control over nanoparticle shape and size is of paramount importance in order to realize the full potential of these novel nanoscale materials.
Thus, there remains a need in the art for a simple method of preparing triangular metal nanoprisms of a predetermined thickness that also provides control over edge length.